Various thermal transfer recording systems are known in the art, and one of them is a thermal dye transfer system in which sublimable dyes as a colorant are thermally transferred from a thermal transfer sheet comprising a substrate sheet, such as a polyester film, bearing the colorants, onto a thermal transfer image-receiving sheet comprising a substrate sheet, such as paper or a plastic film, bearing a dye-receptive layer, thereby forming various full-color images on the thermal transfer image-receiving sheet.
In this case, a thermal head mounted on a printer is used as heating means, and dots of three or four colors are transferred onto the receptive layer of a thermal transfer image-receiving sheet by controlled heating for a very short period of time, thereby reproducing a full-color image of an original utilizing the dots of a plurality of colors.
The image thus formed, since dyes are used as the colorant, has excellent sharpness, transparency, halftone reproduction, and gradation, and the quality thereof is comparable to that of images formed by the conventional offset printing or gravure printing and that of full-color photographic images.
The image-receiving sheet for sublimation transfer has a receptive layer, for receiving a dye, on a substrate. In addition, an intermediate layer formed of a resin having a relatively low glass transition point is formed between the substrate and the receptive layer from the viewpoint of imparting cushioning properties and flexibility to the image-receiving sheet.
However, the formation of an image on an image-receiving sheet having an intermediate layer formed of a resin having a low glass transition point poses a problem that, when the sheet with an image transferred thereon is stored at a high temperature for a long period of time, the dye image is diffused into an intermediate layer to cause a sharp image to bleed or blur.
Further, the intermediate layer should serve to adhere the substrate to the receptive layer. When it is formed of a resin having low adhesion, separation occurs between the substrate and the intermediate layer or between the intermediate layer and the receptive layer in the formation of an image in a printer or during use of a print.